With regard to their operating mode, a distinction is made between free-running and fixedly clocked switching converters, which are fundamentally identical in their construction but differ in respect of the manner of clocked driving of the switch which is connected in series with the inductive energy storage element and controls the power consumption.
Free-running switched-mode power supplies are sufficiently known for the DC voltage supply of loads, such as, for example, computers, monitors, television sets or the like. The basic construction and the method of operation of such a switched-mode power supply are described for example in DE 197 32 169 A1. For driving the switch which is present in such power supply units and controls the power consumption, use is usually made of integrated circuits, such as, for example, drive modules of the TDA 4605 or TDA 16846 type, which are available from the applicant.
In order to provide a better understanding of the invention explained below, firstly the basic construction and the basic method of operation of a conventional free-running flyback converter switched-mode power supply and of a conventional fixedly clocked flyback converter switched-mode power supply will be explained with reference to FIGS. 1 to 3.
The switching converter comprises input terminals EK1, EK2 for the application of a rectified input voltage Uin, and output terminals AK1, AK2 for the provision of an output voltage Uout for a load. A transformer Tr is provided for converting the input voltage Uin into the output voltage Uout, the primary coil Lp of the said transformer being connected in series with a semiconductor switch T1 between the input terminals EK1, EK2, and the secondary coil Ls of said transformer being connected to the output terminals AK1, AK2 via a rectifier arrangement GL. In the case of such a flyback converter switched-mode power supply, the primary coil Lp takes up energy from the input voltage Uin while the switch is closed, and emits this energy to the load via the secondary coil Ls and the rectifier arrangement GL when the switch is subsequently opened.
The task of such power supply units is to keep the output voltage Uout constant in a manner largely independent of fluctuations in the power consumption of the load and the input voltage. A control loop is present for controlling the output voltage or the power consumption of the switched-mode power supply and has a control signal RS which is derived from the output voltage Uout and determines the power consumption of the switched-mode power supply. This control signal RS is fed to a drive circuit 20, which provides a drive signal PS for clocked driving of the switch.
The drive signal PS comprises a sequence of drive pulses, the time duration of the individual drive pulses, that is to say the switch-on duration of the switch, being dependent on the control signal RS, and said time duration rising as the power demand of the load increases, in order to keep the output voltage Uout constant. The points in time at which the switch is closed in a free-running/quasi-resonant switching converter are dependent on the magnetization state of the inductive energy store and usually prescribed by points in time at which the primary coil Lp has emitted the previously stored energy to the secondary side Ls, and is thus demagnetized. Such magnetization states of the primary coil are detected by means of an auxiliary coil Lh, which is coupled to the primary coil and is likewise connected to the drive circuit IC.
By way of example, FIG. 2 shows the temporal profile of a drive signal PS, the power consumption Pin and the magnetization M of the primary coil Lp for a switching converter operated in free-running fashion. These signal profiles are in each case illustrated for a first value of the control signal RS in the left-hand part and for a second value of the control signal RS in the right-hand part. In this case, the second value of the control signal is less than the first value of the control signal, so that the second control signal value results in shorter switch-on durations ton than the first control signal value, as shown in FIG. 2.
After the closing of the switch, an input current Iin flowing through the primary coil rises in each case linearly proceeding from zero. With a constant input voltage Uin, the power consumption Pin is proportional to the current consumption and has the ramp-shaped profile illustrated. In a corresponding manner, the magnetization M rises linearly after the switch-on and falls linearly after the switch-off during the time durations toff, the switch being switched on again in the case of free-running operation when the magnetization has decreased to zero. In this case, the demagnetization time of the primary coil Lp is proportional to the magnetization time.
A switching period of the switch is determined by the time duration between the beginning of two successive switch-on pulses of the drive signal PS. In this case, the energy consumed by the power supply unit is proportional to the area under the curve for the power consumption Pin and is proportional to the area under the curve for the magnetization M. The mean power consumed results from the energy consumed per on/switching period. Assuming that the input voltage Uin is constant for at least a few switching periods, this mean power is proportional to the switch-on duration ton and is thus proportional to the control signal RS. Owing to the proportionality of the demagnetization duration with respect to the switch-on duration, the magnetization durations Tm illustrated in FIG. 2, which results from the switch-on duration and the demagnetization time, are also proportional to the control signal.
In the case of a fixedly clocked operation of the switching converter, the switch T1 is switched on in a fixed clock cycle prescribed by a clock signal Tclk, as is illustrated by way of example in FIG. 3. The switch-on duration ton of the switch T1 is again dependent on the control signal RS, the magnetization profiles of the primary coil Lp for two different large control signal values being illustrated in the left-hand and right-hand parts of FIG. 3. A quadratic dependence between the switch-on duration ton—and thus the magnetization duration Tm and the control signal RS—and the mean power consumption results in the fixedly clocked operation.
The linear dependence of the output power emitted by a switching converter on the control signal in the free-running operation and the quadratic dependence of this power on the control signal are illustrated in FIG. 4, in which the emitted output power is illustrated against the control signal. As is furthermore revealed in FIG. 4, the emitted output power is also dependent on the input voltage in the free-running operation.
One advantage of free-running flyback converter switched-mode power supplies is their high efficiency. They are therefore increasingly being used for compact power supply units in closed plastic housings, because the heat emission that is permissible in the case of such housings is severely limited. Unlike in the case of fixedly clocked switched-mode power supplies, in the case of free-running/quasi-resonant switched-mode power supplies, however, the instantaneous switching frequency changes with the power consumption of the load, the information about this power consumption being fed back to the drive circuit of the switch by means of the control signal. In the case of free-running switching converters, this switching frequency of the switch increases as the power consumption of a connected load decreases, as a result of which the switching losses increase and the efficiency decreases in the case of small power levels to be emitted. Moreover, problems due to EMC radiation increase as the switching frequency of the switch increases.
Solution approaches for avoiding excessively high switching frequencies in the case of small power emissions are described for example in DE 44 37 459 C1, DE 197 32 169 A1, U.S. Pat. No. 6,229,716 or DE 199 39 389 A1. What is common to these solutions is that, in the case of a flyback converter switched-mode power supply, after a magnetization of the primary coil, a waiting time comprising a few periods of a sinusoidal free transformer oscillation following this demagnetization elapses before the switch is closed again. In this case, the number of oscillation periods which the waiting time comprises is dependent on the control signal. What is problematic in this case is that any change in the number of oscillation periods which the waiting time comprises results in an abrupt change in the transfer characteristic of the switching converter with regard to the control signal. Any such abrupt change entails a switch-on process which results at least in a temporary ripple of the output voltage. In the extreme case, these abrupt changes may lead to instabilities in the overall system.
In order to avoid EMC problems DE 102 42 218.4 describes a method for driving a switch in a switching converter, which provides for a plurality of switch-on and switch-off processes to be carried out during a drive period in the free-running operation, the duration of at least one switch-on pulse being modulated from drive period to drive period within a predetermined time range and the switch-on durations of the remaining switch-on pulses within a drive period being coordinated with the time duration of the modulated switch-on pulse such that, with a control signal remaining the same, the mean power consumption per drive period is at least approximately constant. In the case of such a method, a frequency modulation of the switching frequency is achieved in the free-running operation with the load remaining the same, as a result of which EMC problems are reduced.
In order to control the power consumption of a switching converter, U.S. Pat. No. 6,275,018 B1 additionally discloses driving the switch that determines the power consumption in each case by means of drive pulses of the same length, the frequency of these drive pulses over time being dependent on the required power consumption. In this case, on the one hand, the time duration between the individual drive pulses may vary and, on the other hand, in the case of a burst mode in which a number of pulses spaced apart uniformly are generated, the number of drive pulses per burst may vary.
U.S. Pat. No. 6,304,473 describes a drive circuit for driving a switch that controls the power consumption in a switching converter, the drive circuit having a pulse generator, a control circuit which is connected between the pulse generator and the switch to be driven and influences the pulse frequency, and a control circuit which is connected to the pulse generator and influences the shape of the pulse signal.
It is an aim of the present invention to provide a method for operating a switching converter which ensures an effective operation of the switching converter with small losses. It is additionally an aim of the invention to provide a drive circuit for driving a switch in a switching converter which ensures an effective operation of the switching converter.